1. Field of the Invention
The present invention relates to a method and an apparatus for low temperature, continuous drying of temperature sensitive materials, especially granular agricultural pesticides, at atmospheric pressure using radio frequency energy.
2. Description of the Related Art
Many products are sold in granular form. Products such as agricultural pesticide and herbicide products are often sold to the end-user as dry, water-dispersible granules. The chemical compounds having the pesticidal or herbicidal activity contained in these products, however, are typically not water soluble. These products must be formulated with additives to produce water-dispersible granules for the use by the end-user. The active ingredients and the additives which comprise the product formulation are typically manufactured as one or more fine powders. To agglomerate the formulation to form larger particles or granules, suitable for end user application, water and sometimes other solvents are added. This moisture or solvent must be driven out after the granules are formed, without damaging or degrading the product. In the drying step the moisture level must be typically reduced from a range of 5%-30% to below 1%, since the granules can stick together and cause product caking at higher moisture levels.
In a common granulation method, known in the art as pan granulation, water is added to the powder(s) and the mixture tumbled or agitated to form the agglomerates. The water is then driven out of the agglomerated product in the drying step. In an another method of granulation, known as paste extrusion, water is added to the powder formulation and a paste is formed. The paste is then extruded into filaments (or extrudates). These extrudates are subsequently dried and then cut into proper size granules prior to packaging and sale.
Since the active ingredients of these formulations easily degrade when subjected, even for relatively short periods of time, to temperatures above a certain threshold, the drying process must be performed below a specified maximum temperature, typically in the range of 60.degree. C. to 90.degree. C. A common method of drying for either pan granulated or extruded formulations is known as vibrating fluidized bed drying. Vibrating fluidized bed drying is performed by supplying high flow rate of warm air, at a velocity typically exceeding 50 meters (.about.150 feet) per minute, to a bed of the wet product. This high velocity air flow delivers the heat of evaporation to the product, mechanically fluidizes the product bed, and removes the vaporized moisture from the process. Vibration of the whole vessel is used to aid the fluidization of the product bed. The humid and typically dusty exhausted process air is then passed through a dust collector, filtered, refrigerated to remove the moisture, re-heated to the specified process temperature, and then fed back into the process.
Such a process is disadvantageous due to the large volume of air required per unit volume of product. The air handling system for such a process is quite large in physical dimensions, represents a large investment of capital, and, due to its energy consumption, is expensive to operate. The agitation of the product inherent in such a drying process partially breaks up the granules and creates large volumes of fines or dust, which reduces the first pass process yield. These fines must be removed from the air cycle and returned for regranulation.
In many processes involving thermally sensitive materials, such as pharmaceuticals, vacuum drying is used, either with or without the aid of an external heating mechanism. The vacuum lowers the boiling point of water (or other solvent), and the drying can be accomplished at a low temperature. Equipment for vacuum drying, commonly used for relatively high value, thermally sensitive products manufactured in relatively small batches, represents a large capital investment. Vacuum drying is inherently a batch process, while the manufacture of granular agricultural products is inherently a continuous process and thus requires a continuous drying process step.
Dielectric or Radio Frequency (RF) drying has been in use in several industries. For example, it is used for drying of textiles, and foodstuffs, and polymers. In RF drying the wet material is placed an intense electric field at a high frequency (typically 10 MHz to 100 MHz). The dipolar molecules of water are rotated at this frequency. The frictional losses at the molecular level generate heat, which evaporates and move the water molecule to the surface of the product. The conventional theory is that the temperature of the material needs to reach the boiling point of water (or the solvent), in order for this evaporation to take place. This conventional thinking has limited the application of dielectric drying to either temperature insensitive products or in the case of temperature sensitive products, to vacuum drying systems when the maximum allowable temperature is below the boiling point of water (or solvent) at atmospheric pressure.